Cell-Free Massive MIMO Enabled Wireless Communication With UAVs in Underlay Spectrum Access Networks

We investigate the use of cell-free massive multiple input multiple output (MIMO) systems in serving unmanned aerial vehicles (UAVs) and ground user equipments (GUEs) in underlay spectrum access mode by considering UAVs as secondary users and GUEs as primary users. We derive a novel lower bound on complement of interference violation probability (IVP). This result helps determine the power margin required at secondary access points (APs) to meet the IVP constraint. We deduce new analytical expressions for spectral efficiency (SE) and bit error rate (BER) of UAVs and GUEs. This analysis entails finding expectation of secondary AP transmit power based on order statistics of the strongest interference channel between secondary APs and GUEs. We prove that for lower pilot power, a larger power margin is required to keep IVP fixed. We show that for a given IVP, the larger the interference threshold is, the greater (lower) the likelihood for the UAVs (GUEs) to obtain higher SE is. For a given interference threshold, probability that UAVs attain higher SE increases as IVP increases for a fixed number of secondary APs (
$N$
) or as
$N$
increases for a given IVP. Impact of interference threshold and height of UAVs on BER is elucidated.